A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. These target portions are commonly referred to as “fields”.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction “spectrum” from which a property of interest of the target can be determined.
At the same time, the known inspection techniques employ radiation in the visible or ultraviolet waveband. This limits the smallest features that can be measured, so that the technique can no longer measure directly the smallest features made in modern lithographic processes. To allow measurement of smaller structures, it has been proposed to use radiation of shorter wavelengths, similar for example to the extreme ultraviolet (EUV) wavelengths used in EUV lithography. Such wavelengths may be in the range 1 to 100 nm, for example, or 1-125 nm. Part or all of this wavelength range may also be referred to as soft x-ray (SXR) wavelengths. Some authors may use SXR to refer to a narrower range of wavelengths, for example in the range 1-10 nm or 1-20 nm. For the purposes of the present disclosure, these terms SXR and EUV will be used without implying any hard distinction. Metrology using harder x-rays, for example in the range 0.1-1 nm is also contemplated. Examples of transmissive and reflective metrology techniques using these wavelengths in transmissive and/or reflective scattering modes are disclosed in published patent application US2015331336A1. Further examples of metrology techniques and apparatuses using these wavelengths in transmissive and/or reflective scattering modes are disclosed in the published patent applications US2016282282A1, US2017045823A1 and WO2017025392A1 and in the international patent application number PCT/EP2016/080058, not yet published at the present priority date (now published as US2017184981A1). The contents of all these applications are incorporated herein by reference.
Convenient sources of SXR radiation include higher harmonic generation (HHG) sources, in which infrared pump radiation from a laser is converted to shorter wavelength radiation by interaction with a gaseous medium. HHG sources are available for example from KMLabs, Boulder Colo., USA (http://www.kmlabs.com/). Various modifications of HHG sources are also under consideration for application in inspection apparatus for lithography. Some of these modifications are disclosed for example in European patent application number 16198346.5 dated Nov. 11, 2016, not published at the priority date of the present application. Other modifications are disclosed in U.S. patent application Ser. No. 15/388,463 and international patent application PCT/EP2016/080103, both claiming priority from European patent application no. 15202301.6 dated Dec. 23, 2015 also not yet published at the priority date of the present application (now published as US2017184511A1). European patent application no. 16188816.9 dated Sep. 14, 2016, not published at the present priority date, describes the correction of wavefronts in an HHG radiation source to minimize blurring of the measurement spot in an inspection apparatus. The contents of all of these applications are incorporated herein by reference.
A wavefront measurement can be used for example to indicate the ability to focus a light source to a specific spot size and shape. This is important information when a high SXR flux is needed in a confined and well-defined spot, for example for overlay metrology. If such parameters can be measured quickly, the information can be used in a feedback loop for adaptive control of the SXR beam used in metrology, or for improving the results of a metrology measurement.
Measurement of wavefronts in the extreme ultraviolet (EUV) and soft x-ray (SXR) spectral region is challenging because of high absorption by most materials and the difficulty to fabricate focusing optics. A common approach in the EUV wavebands is to use a Hartmann sensor, which is an array of apertures, to measure the local phase gradient. Examples are described is Merc{tilde over (e)}re et al., Opt. Lett. 28, 1534 (2003), in Künzel et al., Appl. Opt. 54, 4745 (2015), and in published patent application US2004196450A1. Another frequently used approach in the EUV is an interferometric technique called phase-shifting point diffraction interferometry (Naulleau et al., Appl. Opt. 38, 7252 (1999)). Two newer, non-standard techniques are also mentioned. The first technique is based on single slit diffraction measured across the beam profile by scanning the slit (Frumker et al., Opt. Lett. 34, 3026 (2009)). The second technique uses the interference pattern between two identical beams, and reconstructs the wavefront by a lateral shearing algorithm (Austin et al., Opt. Lett. 36, 1746 (2011)).
Considering the HHG source which is promising for EUV/SXR metrology, HHG sources are by nature broadband in spectrum and tend to suffer from variations in beam parameters due to the nonlinear generation process. While fast spectrum measurements exist, a fast 2-D wavefront measurement can only be done without spectral resolution, i.e. integrated over the full spectrum of the source. Recent measurements show that there can be significant variation of the wavefront for different harmonics. Thus, there is a desire to measure spectrally resolved 2-D wavefronts on timescales shorter than a typical measurement, and potentially to perform feedback on the HHG source to stabilize its characteristics.
For spectrally resolved wavefront measurements, each of the techniques mentioned in the description of the state-of-the-art has to be combined with an additional spectrometer. This results in a constraint to measure the wavefront only in one dimension because an EUV spectrometer requires one spatial dimension of the camera to record the spectrum. To measure the wavefront with spectral resolution and 2-D would then require a scanning process, which would be slow and cumbersome.
Attempts have been made to resolve spectral components in wavefront sensors, using color filters in the apertures (see for example US2016109290A1 and RU2036491C1). Unfortunately, these techniques reduce the spatial resolution of the sensor in proportion to the number of colors, and color filters are not readily available for EUV/SXR wavelengths.